This invention relates to the few seconds during which an inflatable elastomeric bladder is "shaped" before, during and after it is inserted into a tire in a curing press. More particularly this invention relates to a shaping system in which plural stages are required, and the desired optimum pressure to which the bladder is to be inflated may vary from one cured tire to the next. This optimum pressure is determined by calculation based on several variables, and when reached, is sensed by a pressure transducer (PT) means, so that, for the first time in the art of curing tires with a bladder, production of cured tires free of a "shaping defect", is a reality.
By "shaping" I refer to the initial inflation of the bladder to permit it to press against the inner surfaces of the tire uniformly and thus displace air between the bladder and the tire. It is critical that essentially no air be trapped between the bladder and the tire. Shaping may be done in several stages at pressures ranging from about 2 psig (lb/in.sup.2 gauge) to about 25 psig depending upon the size of the green carcass, and whether it is for a bias or radial tire. For example, for a radial tire, the pressures may be sequentially increased from 2 psig to 5 psig, then to about 15 psig, and held for a different interval of time at each pressure level. The internal pressure during the curing step may likewise be "stepped" and held at each pressure level for a different interval of time which will be a function of the rate at which the desired optimum pressure is reached.
Only those persons actively involved with the curing (vulcanizing) of tires appreciate how easy it is for a scrap tire to be generated, and how high a percentage of such scrap is generated by a shaping defect. Shaping defects are so termed because they are directly attributable to a problem in the sequence of steps, typically during loading, or the first, second and third stages of shaping pressure during shaping.
Shaping defects include "trapped air" between the bladder and the tire, most likely resulting from undershaping. This typically occurs with a start-up of a `cold` press, or when the bladder has just been changed and, being new and stiff, does not inflate as quickly or as well as it will after it is broken in with a dozen or more `heats` (curing cycles).
A "rolled bladder" defect is attributed to the bladder rolling up against the loader paddles during ring drop. This is caused by too high a pressure in the first shaping stage, or using a bladder which is the flaccid survivor of too many heats.
If the shaping pressure in the first stage is either too high or too low, the tire is found to be "cocked in the mold", or incorrect pressure could result in buckling ("buckles") of portions of the bladder which results in defects in the tire. If in addition, the shaping pressure in the third stage is too low, "sidewall lightness" is the result, or if too high, a tire with "tread folds", which indicate "overshaping", is the result.
The simple fact is that any one of the foregoing defects results in a rejected tire and a correspondingly higher cost of producing acceptable tires. Over the years, the effort to minimize shaping defects and enhancing quality, has been unremitting. This effort has included using a wide array of compounds for the specific purpose of producing more reliable bladders; and a number of shaping systems relying on mechanical improvements in the valving, and switching from one type of piping layout to another more symmetrical one, using a carefully regulated low pressure steam supply, and the like. Details about such systems are seldom referred to in the published literature because they are zealously guarded as invaluable know-how.
A typical cycle for a passenger tire is about 15 min. and the sum of the time required for each of the plural shaping stages is generally less than 45 secs. The remaining portion of the cycle consists of the curing portion and the blowdown portion.
The blowdown portion of the cycle is the time it takes to purge steam from the bladder in each mold cavity until the pressure is sufficiently low to open the mold. Depending on numerous factors which are detailed in my copending U.S. patent application Ser. No. 888,896 filed July 24, 1986, this period was fixed in the prior art by a timer for each press and is about 30 sec for a passenger tire. If, for some reason, the pressure in the bladder is not low enough, the press will not open because of a pressure safety switch which mechanically responds to pressure on a diaphragm. The safety switch is set for a pressure low enough so as not to injure an operator who happens to be close by when the press opens. Such pressure is in the range from about 3 psig to about 10 psig.
Each press is already equipped with a mechanical pressure safety switch means, typically a diaphragm type in pressure sensing relationship with the interior of the bladder, which switch is preset for a preselected pressure at which the press may be safely opened. A similar mechanical pressure switch may be used to trigger the ring drop (for radial tires) when the pressure reaches a predetermined level, say 2 psig. Still another mechanical pressure switch, also of the diaphragm type, is used to warm up and pulse a newly installed bladder, or start up a cold press. With these mechanical pressure sensors and their inherent advantage in coping with a power failure, there is no reason to use an electronic pressure sensor. Moreover, since the blowdown sequence and all other curing operations in conventional presses are timer controlled, there is no reason to desire the installation of yet another pressure sensor, whether mechanical or not. Most of all, trying to monitor pressure sensitively enough to control the extent to which a bladder is inflated with a critically sensitive optimum desired pressure, seemed unlikely to become the basis of a successful shaping system.
The curing room typically houses from 50 to about 250 curing presses which are fed by central steam and water systems through steam and water manifolds to which each press is connected with appropriate valving. Of particular concern to this invention is the shaping steam manifold to which each press is connected. This manifold is typically a relatively large diameter steam line, in the range from 2" to 4" diam. depending upon the number of presses commonly manifolded.
Each press may include a single, but usually two, simultaneously operated molds and the press is preferably individually controlled by its own programmable controller ("PC") and the necessary instrumentation and hardware which allows a press to be operated automatically. The shaping instructions, specifically, for the PC, and the curing instructions generally, may be downloaded from a central computer each time the curing cycle is to be changed.
A curing press may be of the `pot-heater` type referred to in U.S. Pat. No. 4,371,483, in which a stack of split-molds loaded with green tires is formed within a pressure vessel closed at the top, with a dome having a butt-plate against which the stack is biased by a hydraulic platform on which the stack rests. Conventional potheaters are of the type manufactured by United McGill of Columbus, Ohio or Pennsylvania Engineering Corp. of Newcastle, Pa., inter alia.
Another curing press is made by McNeil Corp. under the Bag-O-Matic trademark. This press uses a bladder which is raised and lowered on a central shaft axially disposed within a green tire in a mold cavity. Still another popular press is made by NRM Corp. under the Autoform trademark. This latter press is also referred to as the "bag-well" press because it uses a bag (bladder) in a central well, and a ram pushes the bag down before a cured tire is removed.
Regardless of the type of curing presses used in the curing room, each is manifolded to a common shaping steam manifold; though each press in a present-day curing room is controlled by a PC, in the prior art, the shaping pressures to which the bladder is inflated for each stage is fixed; that is, control valves, or pressure controllers (like those made by Sinclair Collins) set predetermined fixed pressures, and establish when each shaping stage is completed. The specific pressures are fixed from experience, and by trial and error.
At one time a Mercoid was used only as a pressure check at the press pause height. Later the Mercoid was used to control over the entire closing sequence instead of just at the press pause height. In addition, the Mercoid range settings were used to pulsate shaping from a low pressure setting to a high pressure setting, oscillating therebetween, through the entire shaping sequence.
More recent shaping systems use plural separate pressure controllers like Jordan, Leslie, Fisher, Sinclair Collins, for each side of the press which allow multiple different pressure settings to be used at different points in the press closing cycle. These provide an advantage over a Mercoid which permitted only one pressure setting. The point at which each shaping stage is initiated is determined by a cam setting on a cam train because a pressure controller does not know the position of the press which may be anywhere between being open and closed. The cam train is a device which mechanically senses the position of the press during the closing sequence. By the positioning of an individual cam, it can electronically transfer control from one shaping stage to another.
After a bladder has been in service for some time, or, when a press is being started up, or, when a ruptured old bladder is replaced with a new one, the pressures are re-set, that is, manually readjusted, again by trial and error.
In the curing of bias tires in some early attempts, it was concluded that the precise pressure for each stage need not be set if the precise location (within the mold) of the inner circumferential edge of the bladder, was sensed with a sensitive mechanical "finger" or a photocell (photoeye). Except that such mechanical sensing means is inapplicable to radial tires.